Transmitter

ABSTRACT

A transmitter comprises a device ( 20 ) for frequency up-conversion of a communication signal having at least two consecutive frequency conversion stages, a power amplifier ( 27 ) for amplifying the output signal therefrom for obtaining a transmission signal and means ( 29 ) for extracting the transmission signal. It also has a device for frequency down-conversion of said extracted transmission signal to the same frequency as the communication signal before said up-conversion for creating a feedback signal. Said two devices use the same local oscillators ( 25, 26 ). The transmitter also has means for signal parameter adaptation of the communication signal on basis of the feedback signal for obtaining a desired character of the transmission signal. The frequency down-conversion device comprises a local oscillator combiner ( 30 ) producing a combined frequency signal used for frequency down-conversion of the transmission signal reducing the number of conversion stages in the path of the extracted transmission signal with respect to the corresponding number of the frequency up-conversion device.

THE BACKGROUND OF THE INVENTION AND PRIOR ART

[0001] The present invention relates to a transmitter according to thepreamble of claim 1, a method for signal processing in a transmitteraccording to the preamble of claim 22, a computer program according toclaim 39 and a computer readable medium according to claim 41.

[0002] “Transmitter” is here to be given a very broad sense and alsocovers so called transmitter configurations having for example aplurality of branches for said communication signal and/or means fortransmitting the transmission signal, such as antennas, as for examplean adaptive antenna. However, it is pointed out that the transmissionmedium does not necessarily have to be “the air”, but it could be anyconceivable medium, such as a cable. Furthermore, “communication signal”is defined as the signal including the information to be transmitted bythe transmitter travelling to and past the power amplifier, and the“transmission signal” is defined as the signal at the output of saidpower amplifier transmitted by the transmitter and also extracted forcreating the feedback signal and travelling in a feedback loop back tosaid means for signal parameter adaptation.

[0003] A transmitter of this type may find may preferred applications,among which as a transmitter for mobile base stations may be mentionedas a non-limitative example.

[0004] The general construction of such a transmitter according to theprior art is illustrated in the appended FIG. 1 and will now be brieflyexplained with reference made thereto. This transmitter has a signalsource 1, which can be digital or analog, a frequency up-conversiondevice 2, a power amplifier 3 and an antenna 28 adapted to transmit thetransmission signal resulting at the output of the power amplifier 3.The frequency up-conversion device has preferably at least two mixingstages each including a local oscillator 4, 5 and a frequency mixer 6,7. Filters 8, 9 are arranged in the signal path downstream of eachfrequency mixer for rejecting unwanted spurii and only letting forinstance the wanted sideband through. The respective filter requirementsmay be reduced if the frequency mixer in question is instead a singlesideband mixer or quadrature modulator configuration having then twofrequency mixers. The reason for using multiple stages in the frequencyup-conversion device is to spread the filtering requirements betweendifferent mixing stages, such that the overall filtering requirementscan be met. If instead only one mixing stage was used, the filterthereof would have to reject all unwanted spurii, some of them veryclose to the wanted signal. Such a filter is very hard to design,especially at high frequencies. With more than one mixing stage, thefiltering requirements will be distributed and therefore be easier met,so that filters with high selectivity work at lower frequencies makingthe design thereof much easier.

[0005] For some transmitters high demands on certain properties of thetransmission signal are put, and they therefore require a feedbackobservation signal from the output of the power amplifier for enablingadjustment of the communication signal for obtaining these properties ofthe transmission signal. The transmission signal is for this sakeextracted at the output of the power amplifier 3 by an extracting means10, which may be for example a coupler, but it may also be for examplean antenna receiving the transmission signal transmitted by the antenna28. A frequency down-conversion of this extracted transmission signalthen takes place in a frequency down-conversion device 11 using the samelocal oscillators 4, 5 as the frequency up-conversion device 2 makingthe design coherent. A design having separate local oscillators for thefrequency up- and down-conversion would also be coherent if the localoscillators were locked to the same time reference. This also means alower cost, since the number of local oscillators is reduced. A feedbacksignal having the same frequency as the input signal to the frequencyup-conversion device is in this way obtained, and this signal is thencompared with a communication signal corresponding to a wantedtransmission signal for signal parameter adaptation of the communicationsignal. The comparison has to be made in this low frequency region, butthe adjustments of the communication signal may be carried out anywherealong the path thereof, such as in this low frequency region, or evenbetween the frequency up-conversion device and the power amplifier orafter the latter. A block 84 between the coupler 10 and antenna 28 isalso shown. This block may have nothing in it, or it may have a duplexerif a receiver is to be attached to the same antenna, or it may have acirculator if it is desired to protect the integrity of the feedbackdown-conversion signal from external signals picked up by the antenna ora combination of the two.

[0006] Said comparison may be made continuously, but it is underlinedthat the signal parameter adaptation is only carried out as often asrequired when the condition changes for creating a transmission signalhaving the desired properties for these new conditions. In one possibleapplication a predistortion of the communication signal, in the digitalor in the analogous domain, is carried out for cancelling distortions ofthe signal generated in the communication signal path, such as in thepower amplifier, for obtaining a substantially distortion-freetransmission signal. These distortions may change with for exampletemperature and component ageing, so that an adaptation of thepredistortion parameters therefore is needed. Another possibleapplication is in a transmitter comprising an adaptive antenna, in whichsaid comparison is utilized to influence the communication signal formeeting gain and/or phase requirements of the transmission signals fromdifferent antennas thereof. Such a transmitter comprises a plurality ofbranches each including a frequency up-conversion device, poweramplifier and an antenna, but the invention also covers the case of sucha multiple branch configuration in which all the branches have oneup-converter in common, or one up-converter and one power amplifier incommon.

[0007] A problem with a transmitter of the type illustrated in FIG. 1 isthat the transmission signal down-conversion path is unnecessarilycomplex and contains a lot of components in the signal path. This leadsto linear as well as non-linear distortion of the signal, distorting thefeedback-signal so created at the output of the frequencydown-conversion device. This makes it troublesome to achieve an accuratecopy of the signal at the output of the power amplifier for saidcomparison reducing the accuracy thereof and thereby the success of theperformance-enhancing adjustments of the transmission signal.

[0008] U.S. Pat. No. 4,700,151 describes a modulation system capable ofimproving a transmission system, in which a feedback signal is createdthrough frequency down-conversion of an output signal from a poweramplifier for predistortion of a communication signal in thecommunication signal path for compensating for non-linearities of theamplified output signal. This transmitter uses the same local oscillatorfor the up- and down-conversion. However, this transmitter only uses onemixing stage.

[0009] Furthermore, U.S. Pat. No. 5,974,302 discloses a transceiver, inwhich the receiver and the transmitter uses the same local oscillators,and the receiver as well as the transmitter thereof have more than onefrequency conversion stage. U.S. Pat. No. 5,937,011 describes atransceiver including dual stage up- and down-conversion and commonlocal oscillators as defined in the preamble of appended claim 1.

SUMMARY OF THE INVENTION

[0010] The object of the present invention is to provide a transmitterand a method for signal processing in a transmitter being improved withrespect to the prior art solution discussed above.

[0011] This object is according to one aspect of the invention obtainedby providing such a transmitter in which the frequency down-conversiondevice comprises a local oscillator combiner having means for mixingfrequency signals of at least two of said local oscillators forcombining them into a combined local oscillator frequency signal, saidlocal oscillator combiner being adapted to send this combined frequencysignal to the frequency mixer of the frequency down-conversion devicefor frequency down-conversion of the transmission signal reducing thenumber of frequency conversion stages in the transmission signal path ofthe frequency down-conversion device with respect to the number offrequency conversion stages in the communication signal path of thefrequency up-conversion device. A reduced number of frequencydown-conversion stages may be used in the frequency down-conversiondevice with respect to the frequency up-conversion device because thetransmission signal at the output of the power amplifier does notcontain any strong interferers that need to be filtered away, but onlythe signal to transmit. This lack of strong interferer means that thereis no need for sharp channel select filters for removing stronginterferer signals. (Of course a block 84 as described above may stillbe needed, for instance for protecting the integrity of the feedbackdown-conversion signal from external signals picked up by an antennaconnected to the power amplifier). This results in fewer components inthe transmission signal path of the frequency down-conversion device andhence lower linear and non-linear distortion of the feedback signal. Aconsequence of this will be a more accurate feedback signal for saidcomparison. Accordingly, the filtering requirements are reduced in saidfeedback transmission signal path. The number of frequency mixers is notreduced, but components are moved away from the feedback transmissionsignal path making this less corruptive.

[0012] According to a very preferred embodiment of the invention saidfrequency up-conversion device has two frequency conversion stages andthe frequency down-conversion device has one single frequency conversionstage in the transmission signal path, and according to yet anotherpreferred embodiment of the invention defined in claim 3 a transmitterhaving three frequency conversion stages in the frequency up-conversiondevice and only one frequency conversion stage in the frequencydown-conversion device is provided. Using one single frequencyconversion stage in the transmission signal path reduces the number ofcomponents therein and linear and non-linear distortions of the outputsignal from the frequency down-conversion device to a minimum.

[0013] According to another preferred embodiment of the invention thetransmitter comprises delay means adapted to delay a frequency signalemitted by at least one of said local oscillators on the path to saidfrequency down-conversion stage of the frequency down-conversion devicefor substantially matching the delay of this frequency signal from saidlocal oscillator to this stage through said local oscillator combinerwith a delay of this signal to this stage through the frequencyup-conversion device and the power amplifier and coupler. Such a delaymatching will remarkably reduce the phase noise contributions of thelocal oscillators to the output signal of the frequency down-conversiondevice. This results in a more accurate feedback signal and/or a fasterobtaining thereof, since such phase noise contributions have otherwiseto be removed through, for example, a time consuming formation of meanvalues.

[0014] According to another preferred embodiment of the invention thetransmitter comprises means for predistortion of said communicationsignal on the basis of said signal parameter adaptation for cancellingout distortions thereof generated in the path of the communicationsignal, such as in the power amplifier, for obtaining a substantiallydistortion-free transmission signal. Said predistortion is preferably,but not necessarily, carried out in the digital domain. Suchpredistortion may be carried out with a high accuracy, since the lownumber of parts in the transmission down-conversion signal path willresult in a low linear and non-linear distortion of the feedback signalused to correct for linear and non-linear distortions in thecommunication signal path.

[0015] According to another preferred embodiment of the invention thetransmitter comprises at least two communication signal branches havinga said frequency up-conversion device each or in common, a poweramplifier each or in common and a said means for extracting thetransmission signal, and said means for signal parameter adaptation isadapted to adjust the gain and/or phase of said transmission signal fromthe respective branch on the basis of a comparison of a communicationsignal corresponding to a wanted transmission signal and said feedbacksignal of each branch or a comparison of feedback signals from allbranches. A transmitter in the form of such a transmitter configurationwill be especially advantageous for use in a transmitter comprising anadaptive antenna, and in which each branch then will have an antenna fortransmitting the transmission signal from the power amplifier. Asimplified and advantageous design of such a transmitter has only onesaid frequency down-conversion device and comprises switching meansadapted to alternatingly connect said frequency down-conversion deviceto different branches for signal parameter adaptation of thecommunication signals thereof. This keeps the costs for components inthe frequency down-conversion part of the transmitter down.

[0016] The invention also relates to a radio base station, acommunication device for communication with a radio base station, awireless Local Area Network, wireline transmitters, e.g. ADSL(Asymmetric Digital Subscriber Line) or a future radio base stationapplications, such as Multiple Input Multiple Output Antenna systems,including a transmitter according to the present invention.

[0017] The invention also comprises a method for signal processing in atransmitter having the features listed in the appended claim 22. Meritsof this method and preferred embodiments thereof defined in thedependent method claims appear clearly from the discussion above ofpreferred embodiments of the transmitter according to the presentinvention.

[0018] The invention also relates to a computer program and a computerreadable medium according to claims 39 and 41. It is easily understoodthat the method defined in the appended set of method claims is verysuitable to be carried out through program instructions from a processorthat may be influenced by a computer program provided with the programsteps in question. This computer program may very well be at leastpartially provided through a network as the Internet.

[0019] Further advantages and advantageous features of the inventionappear from the following description and the other dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] With reference to the appended drawings, below follows a specificdescription of preferred embodiments of the invention cited as examples.

[0021] In the drawings:

[0022]FIG. 1 is a simplified block diagram of a transmitter according tothe prior art,

[0023]FIG. 2 is a block diagram of the frequency up- and down-conversionpart of a transmitter according to a first preferred embodiment of theinvention,

[0024]FIGS. 3a and 3 b are simplified frequency diagrams used to explainthe function of different frequency conversion stages in a transmitteraccording to the invention,

[0025]FIG. 4 is a block diagram corresponding to FIG. 2 of a transmitteraccording to a second preferred embodiment of the invention,

[0026]FIG. 5 is a block diagram corresponding to FIG. 2 of a transmitteraccording to a third preferred embodiment of the invention,

[0027]FIG. 6 is a simplified block diagram substantially correspondingto FIG. 1 of a transmitter in the form of a transmitter configurationaccording to a fourth preferred embodiment of the invention, and

[0028]FIG. 7 is a flow chart illustrating a general method for signalprocessing in a transmitter according to a preferred embodiment of theinvention.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

[0029] Firstly, a part of a transmitter according to the presentinvention, which is not specific for this invention will be describedwith reference to the prior art transmitter according to FIG. 1, andthis part of the transmitters according to different preferredembodiments of the invention illustrated in FIGS. 2, 4 and 5 has beenleft out in these Figures. Usual data to be transmitted by thetransmitter, for instance emanating from human speech, are provided at 1and sent on to a base band-processing unit 12. The output therefrom istreated in a data modification unit 13 with the aim to obtain enhancedperformance on the transmission signal obtained further downstream atthe output of the power amplifier 3. The signal is then sent to adigital quadrature modulator 14 combining the complex base-band signalwith a digital quadrature oscillator signal into a digital intermediatefrequency signal, which is converted into an analog signal in adigital/analog-converter 15 and sent on to the analog frequencyup-conversion device 2.

[0030] The output signal from the frequency down-conversion device 11 isconverted into the digital domain by an analog/digital converter 16 andthen processed in a digital quadrature demodulator 17 for the purpose ofbeing compared with a communication signal corresponding to a wantedtransmission signal at the output of the power amplifier 3 in aparameter adaptation means 18 influencing the data modification in theunit 13. The members described in these two paragraphs may be the samein a transmitter according to the invention. The prior art transmitteralso has a delay means 19 arranged to match the time delay of thecommunication signal for said comparison with the feedback signal.

[0031] The most important features of a first preferred embodiment of atransmitter according to the present invention will now be describedwhile simultaneously referring to FIG. 2 and FIGS. 3a and 3 b. 20illustrates a frequency up-conversion device functioning in the same wayas described above with reference to FIG. 1 including an intermediatefrequency mixer 21, a filter and amplifier block 22, a radio frequencymixer 23 and a filter block 24. 25 and 26 stand for local oscillators inthe form of an intermediate frequency oscillator and a radio frequencyoscillator, respectively. 27 and 28 are the power amplifier and anantenna, respectively. Means for extracting a transmission signalresulting at the output of the power amplifier 27 is indicated at 29.

[0032] The frequency down-conversion device of this transmittercomprises a local oscillator combiner 30, having means in the form of alocal oscillator frequency mixer 31 for mixing the frequency signalsfrom the two local oscillators 25 and 26 for combining them into acombined local oscillator frequency signal. The local oscillatorcombiner also comprises two blocks 32, 33 containing amplification meansfor driving the mixer 31 and possibly filters or circulators. Thecombiner further comprises a block 34 for filtering and amplifying thecombined local oscillator frequency signal to drive a frequency mixer 35of the frequency down-conversion device for frequency down-conversion ofthe transmission signal. A filter and amplifier block 36 is indicated atthe output of the frequency down-conversion mixer 35.

[0033] It is also illustrated how the transmitter may comprise differentdelay means 37-39 for cancelling phase noise from the local oscillators25 and 26. More exactly, the phase noise introduced through the localoscillator 25 is cancelled if the delay of a signal from this localoscillator to the frequency down-conversion stage 35 of the frequencydown-conversion device through said local oscillator combiner 30, i.e.on the path 40, is matched with the delay of this signal to this stagethrough the frequency up-conversion device and the power amplifieraccording to the path 41. The same is valid for the local oscillator 26and the paths 42 and 43, respectively. Phase noise contributions of thelocal oscillators 25 and 26 to the feedback signal resulting at 44 mayin this way be remarkably reduced. This means a faster signal parameteradaptation, since for example no time consuming mean value formation ofthe feedback signal emanating from the extracted transmission signalwill be needed for cancelling phase noises out. However, one or more ofthe delay means 37-39 may be left out should the local oscillators havesufficient noise performance. It may also be possible to leave out adelay means because the difference in delay between two signal paths issmall enough. This may for instance be the case for the local oscillator26 and the paths 42 and 43.

[0034] The function of the part of the transmitter illustrated in FIG. 2with respect to the frequency mixing will be as follows: A communicationsignal including user data having a local oscillator communicationfrequency LO_(COM) arrives to the intermediate frequency mixer 21 fromthe digital/analog-converter 15 and is there mixed with an intermediatefrequency signal from the local oscillator 25 having a local oscillatorintermediate frequency LO_(IF) resulting in a lower sideband L and anupper sideband U having a frequency of LO_(IF)−LO_(COM) andLO_(IF)+LO_(COM), respectively, as illustrated in FIG. 3a. It isschematically illustrated through a dashed line w how a filter functionis applied on the signal from the mixer 21 in the filter 22 forfiltering out the upper sideband U sent further to the radio frequencymixer 23, but it is pointed out that any of the two sidebands may beselected and sent on to the mixer 23, where it is mixed with a radiofrequency signal from the local oscillator 26 having a local oscillatorradio frequency LO_(RF), which results in a signal having a lowersideband L and an upper sideband U, respectively. The lower sidebandwill have a frequency of LO_(RF)−LO_(IF)+LO_(COM) orLO_(RF)−LO_(IF)−LO_(COM) depending upon whether the lower or uppersideband had been filtered out in the filter 22. The upper sideband willhave a frequency of LO_(RF)+LO_(IF)−LO_(COM) or LO_(RF)+LO_(IF)+LO_(COM)depending upon whether the lower or upper sideband has been filtered outin the filter 22. One of these sidebands are here filtered out throughthe filter 24 and amplified through the amplifier 27, so that thetransmission signal being frequency modulated to one of said fourfrequencies is transmitted by the antenna 28.

[0035] In a corresponding way frequency signals from the two localoscillators 25 and 26 having a frequency of LO_(IF) and LO_(RF),respectively, are mixed in the frequency mixer 31, and a lower sidebandhaving a frequency of LO_(RF)−LO_(IF) and an upper sideband having afrequency of LO_(RF)+LO_(IF) are formed. One of these frequencies isfiltered out in the block 34. This combined local oscillator frequencysignal is mixed in the frequency mixer 35 with the extractedtransmission signal so that a lower sideband L having a frequency ofLO_(COM) is achieved as schematically illustrated in FIG. 3B. Moreexactly, the upper sideband LO_(RF)+LO_(IF) resulting from the combinedlocal oscillator frequency signal is applied if LO_(RF)+LO_(IF)+LO_(COM)or LO_(RF)+LO_(IF)−LO_(COM) has been selected by the filters 22 and 24as the transmission signal frequency and LO_(RF)−LO_(IF) is applied ifany of the two other possible frequencies of the transmission signalreferred to above have been filtered out by the filters 22 and 24. As anon-limitative example it may be mentioned that LO_(COM) may be 12.5MHz, LO_(IF) 128 MHz and LO_(RF) 1992 MHz. If then the upper sidebandwill be filtered out by the filter 22 this will have a frequency of140.5 MHz, and if the upper sideband again filtered out through thefilter 24, this will have a frequency of 2132.5 MHz. If the extractedtransmission signal for this frequency then is mixed with the uppersideband resulting in the frequency mixer 31 having a frequency of 2120MHz the lower sideband so obtained will have a frequency of 12.5 MHz.

[0036] It is pointed out that the frequency values just mentioned areonly examples of typical frequencies used for frequency modulation inradio base stations, and any other frequencies may be envisaged. Afilter window function w is then applied on this signal through thefilter 36 for filtering out said lower sideband L and convey thefeedback signal so created to the analog/digital-converter 16 for use inthe comparison for signal parameter adaptation in 18. As stated above,it is possible to use a single frequency conversion stage in the path ofthe extracted transmission signal in the feedback loop because thesignal on the output of the power amplifier 27 does not contain anyinterferers that need to be filtered away, but only the signal to betransmitted. The lack of a strong interferer means in the practice thatthere is no need for sharp channel select filters for removing stronginterferer signals. This leads to fewer and cheaper parts in the signalpath and also to lower linear and non-linear distortion of the feedbacksignal so created. It is noticed that the number of frequency mixingstages and filters in the frequency down-conversion device is notreduced with respect to the prior art transmitter shown in FIG. 1, but apart of them is removed from the path of the extracted transmissionsignal, so that it will be fewer components corrupting the feedbacksignal in this path increasing the accuracy of this signal and of thesignal parameter adaptation.

[0037] A transmitter according to a second preferred embodiment of theinvention being slightly modified with respect to the transmitterillustrated in FIG. 2 is schematically shown in FIG. 4. Partscorresponding to parts of the embodiment according to FIG. 2 areprovided with the same reference numerals. It is illustrated how thefrequency mixer 21 is replaced by an analog quadrature modulator 45containing two frequency mixers and making the filter 22 simpler.However, an amplifier and filters may of course be arranged between thequadrature modulator 45 and the frequency mixer 23, although omitted inthe Figure. The frequency down-conversion stage is here carried outresulting in baseband I and Q feedback signals at the output thereof. Asan alternative an SSB (Single Side Band) mixer may be used instead ofthe quadrature modulator and/or the quadrature demodulator.

[0038]FIG. 5 illustrate a transmitter according to a third preferredembodiment of the invention having three frequency up-conversion stagesand one single frequency down-conversion stage in the path of theextracted transmission signal. 47-49 correspond to frequency mixers,50-52 correspond to filter and amplifier blocks and 53-55 correspond tolocal oscillators. It is illustrated how the frequency signals from thelocal oscillators 53 and 54 are combined in a local oscillator combiner56 for creating a first combined local oscillator frequency signal,which is then mixed with the frequency signal from the local oscillator55 in a second local oscillator combiner 57 for forming a secondcombined local oscillator frequency signal to be mixed with theextracted transmission signal in the down-conversion mixer 58. 59 and 60are the power amplifier and the antenna, respectively. Although thisembodiment may in some applications be preferable, it is pointed outthat in general the present invention will loose in interest with anincrease of the number of the local oscillators used, since the localoscillator combiner configuration will grow in complexity. It is withinthe scope of the present invention to reduce the number of frequencyconversion stages in the path of the extracted transmission signal withrespect to the frequency conversion stages in the frequencyup-conversion device arbitrarily, so that in the case of three localoscillators as in FIG. 5 the number of frequency down-conversion stagesin the path of the extracted transmission signal may be reduced to two,but that would introduce more linear and non-linear distortion to thefeedback signal.

[0039]FIG. 6 illustrates a transmitter in the form of a transmitterconfiguration according to a fourth preferred embodiment of theinvention in an Adaptive Antenna application. Adaptive or smart antennasare used for transmitting signals in different directions by modifyingthe amplitude and/or phase in each branch and/or communication signals.It is then important that the requirements of gain and phase accuracy ofthe transmission signals are met, which may be achieved by using afeedback signal as described above. In a radio base stations therequired gain and phase accuracy may typically be 0.2 dB and 1°,respectively.

[0040] This transmitter comprises a number of communication signalbranches with a signal source 61, base band processing unit 62, datamodification unit 63, digital/analog-converter 64, frequencyup-conversion device 65, power amplifier 66 and antenna 67. In this caseof separate sources 61 for each branch the data modification unit 63 isa matrix, that is every input is able to map to every output asillustrated through the dashed line 85. Each branch may have a frequencydown-conversion device of the type shown in FIG. 2, but there may alsobe a frequency down-conversion device 68 in common to all branches asillustrated in FIG. 6. This means that switching means 69 are providedfor alternatingly connect the frequency down-conversion device 68 todifferent antenna branches 70 ₁-70 _(N) for signal parameter adaptationof the communication signals thereof. All frequency up-conversiondevices may share the same local oscillators 71, 72, but it is alsopossible that they have separate local oscillators requiring switchingmeans adapted to alternatingly connect the local oscillators ofdifferent frequency up-conversion devices with the local oscillatorcombiner 73 of the frequency down-conversion device. What thesedifferent alternatives would look like is apparent to a man with skillin this art and they have therefore not been illustrated by drawings forkeeping the number of drawings on a reasonable level.

[0041] Furthermore, the feedback signal will be compared in a signalparameter adaptation means 74 with a communication signal correspondingto a wanted transmission signal for the respective branch or allfeedback signals are compared if only the wanted relative phase and gainof the individual transmission signals are needed. A signal parameteradaptation is then carried out on the basis of this comparison, in thefirst place for adjusting the gain and phase of the transmission signalfrom the respective branch, but also for applying predistortion to therespective communication signal for cancelling out distortions thereofgenerated in the path of the communication signal, such as in the poweramplifier 66, for obtaining a substantially distortion-free transmissionsignal of the respective branch if desired. The adjustments may as analternative be carried out on the analog side of the communicationsignal path. Furthermore, shown are separate sources 61, it couldpossibly be just one source which is eventually split. This splittingjunction can be anywhere from the source to the antenna, assuming thatthe gain and/or phase adjustment means is after the point of splittingbut not necessarily directly after the splitting junction. Accordingly,the separate branches may share the same up-conversion device, and thismay as an alternative be achieved by a switching means similar toswitching means 69.

[0042]FIG. 7 shows a flow chart very schematically illustrating steps inthe frequency up- and down-conversion part of a method according to thepresent invention. It is started with a frequency signal at 75 beingtwice frequency up-converted 76, 77 and then amplified 78 for beingtransmitted 79 and extracted 80 to be frequency down-converted 81 aftera local oscillator combining 82 in a parallel branch. A feedback signalis then obtained at the end 83.

[0043] The invention is of course not in any way restricted to thepreferred embodiments described above, but many possibilities tomodifications thereof may be envisaged by a man with ordinary skill inthe art without departing from the scope of the invention as defined inthe appended claims.

[0044] It is pointed out that the invention is directed to transmittershaving an up-conversion device and a down-conversion device sharinglocal oscillators, in which local oscillators are combined for reducingthe number of conversion stages in the down-conversion device withrespect to the number of such stages in the up-conversion device.However, to use separate local oscillators for the up- anddown-conversion for having fewer down-conversion stages is not claimedhere, although realized by the present inventors.

1. A transmitter comprising a device for frequency up-conversion (20,65) of a communication signal to a higher frequency comprising at leasttwo consecutive frequency conversion stages each having a localoscillator (25, 26, 53-55) and a frequency mixer (21, 23, 47-49) formixing the signal of the local oscillator with said communication signalfor a stepwise frequency up-conversion of the communication signal, apower amplifier (27, 59, 66) for amplifying the output signal from thefrequency up-conversion device for obtaining a transmission signal,means (29) for extracting the transmission signal, a device forfrequency down-conversion of said extracted transmission signal to thesame frequency as the communication signal before said up-conversion forcreating a feedback signal, said frequency down-conversion device beingadapted to use the same local oscillators as the frequency up-conversiondevice for frequency conversion of the transmission signal, and means(18, 74) for signal parameter adaptation of the communication signal onbasis of said feedback signal for obtaining a desired character of saidtransmission signal, characterized in that said frequencydown-conversion device comprises a local oscillator combiner (30, 56,57, 73) having means (31) for mixing frequency signals of at least twoof said local oscillators for combining them into a combined localoscillator frequency signal, and that said local oscillator combiner isadapted to send this combined frequency signal to a frequency mixer (35,46, 58, 68) of the frequency down-conversion device for frequencydown-conversion of the transmission signal reducing the number offrequency conversion stages in the transmission signal path of thefrequency down-conversion device with respect to the number of frequencyconversion stages in the communication signal path of the frequencyup-conversion device.
 2. A transmitter according to claim 1,characterized in that said frequency up-conversion device has twofrequency conversion stages and the frequency down-conversion device hasone single frequency conversion stage in the transmission signal path.3. A transmitter according to claim 1, characterized in that thefrequency up-conversion device has three frequency conversion stages,that the frequency down-conversion device comprises two said localoscillator combiners (56, 57), namely a first one (56) with means formixing frequency signals from two (53, 54) of said local oscillators forcombining them into a first combined local oscillator frequency signaland a second (57) with means for mixing said first combined localoscillator frequency signal with a frequency signal from a third (55) ofsaid local oscillators for combining them into a second combined localoscillator frequency signal, and that said second local oscillatorcombiner is adapted to send this second combined local oscillatorfrequency signal to a frequency mixer (58) of the frequencydown-conversion device for frequency down-conversion of the transmissionsignal reducing the number of frequency conversion stages in thetransmission signal path of the frequency down-conversion device to one.4. A transmitter according to any of claims 1-3, characterized in thatone of the frequency conversion stages in the frequency up-conversiondevice is formed by a quadrature modulator (45) and said frequencyconversion stage of the frequency down-conversion device for frequencydown-conversion of the transmission signal is formed by a quadraturedemodulator (46).
 5. A transmitter according to any of claims 1-3,characterized in that at least one of the frequency conversion stages inthe frequency up-conversion device and/or said frequence conversionstage of the frequency down-conversion device for frequencydown-conversion of the transmission signal is formed by a SSB (SingleSideBand) mixer.
 6. A transmitter according to any of the precedingclaims, characterized in that it comprises delay means (37-39) adaptedto delay a frequency signal emitted by at least one of said localoscillators on the path to said frequency down-conversion stage of thefrequency down-conversion device for substantially matching the delay ofthis frequency signal from said local oscillator (25, 26) to this stagethrough said local oscillator combiner (30) with the delay of thissignal to this stage through the frequency up-conversion device (20) andthe power amplifier (27).
 7. A transmitter according to claim 6,characterized in that it comprises a said delay means (37, 38) for eachof said local oscillators (25, 26).
 8. A transmitter according to claim6 or 7, characterized in that it comprises a said delay means (39)connected between the frequency mixing means (31) of said localoscillator combiner (30) and said transmission signal frequencydown-conversion stage of the frequency down-conversion device.
 9. Atransmitter according to any of claims 6-8, characterized in that itcomprises at least one said delay means (37-39) arranged to delay afrequency signal sent from one of said local oscillators (25, 26) tosaid local oscillator combiner (30).
 10. A transmitter according to anyof the preceding claims, characterized in that it comprises means (13)for predistortion of said communication signal on the basis of saidsignal parameter adaptation for cancelling out distortions thereofgenerated in the path of the communication signal, such as in the poweramplifier (27, 59, 66), for obtaining a substantially distortion-freetransmission signal.
 11. A transmitter according to claim 10, charactrized in that it comprises an analog/digital-converter (16) adapted toconvert the transmission signal into a digital signal, and that saidpredistortion means (13) is adapted to carry out said predistortion ofthe communication signal in the digital domain.
 12. A transmitteraccording to any of the preceding claims, characterized in that itcomprises at least two communication signal branches (70 ₁-70 _(N))having a said frequency up-conversion device (65) each or in common, apower amplifier (66) each or in common and a said means for extractingthe transmission signal, and that said means (63) for signal parameteradaptation is adapted to adjust the gain and/or phase of saidtransmission signal of the respective branch on the basis of acomparison of a communication signal corresponding to a wantedtransmission signal and said feedback signal of each branch or acomparison of feedback signals from all branches.
 13. A transmitteraccording to claim 12, characterized in that it comprises a separatesaid frequency down-conversion device (68) for each of said branches (70₁-70 _(N)).
 14. A transmitter according to claim 12, characterized inthat it has only one said frequency down-conversion device (68), andthat it further comprises switching means (69) adapted to alternatinglyconnect said frequency down-conversion device to different branches (70₁-70 _(N)) for signal parameter adaptation of the communication signalsthereof.
 15. A transmitter according to claim 13 or 14, characterized inthat all said frequency up- and down-conversion devices (65, 68) sharethe same local oscillators (71, 72).
 16. A transmitter according toclaim 14, characterized in that it comprises a separate frequencyup-conversion device for each branch and all frequency up-conversiondevices (65) have separate local oscillators, and that the transmittercomprises switching means adapted to alternatingly connect the localoscillators of different frequency up-conversion devices with the localoscillator combiner (73) of the frequency down-conversion device.
 17. Atransmitter according to any of claims 12-16, characterized in that itcomprises an adaptive antenna, and that each branch (70 ₁-70 _(N)) hasan antenna (67) for transmitting the transmission signal from the poweramplifier (66).
 18. A radio base station including a transmitteraccording to any of claims 1-17.
 19. A communication device, such as atelephone, for communication with a radio base station and including atransmitter according to any of claims 1-17.
 20. A wireless Local AreaNetwork including a transmitter according to any of claims 1-17.
 21. Awireline network, such as an ADSL network or a Multiple Input MultipleOutput Antenna system, including a transmitter according to any ofclaims 1-17.
 22. A method for signal processing in a transmittercomprising the following steps: a) frequency up-conversion of acommunication signal to a higher frequency by consecutively mixing thecommunication signal with a signal from at least two consecutive localoscillators (25, 26, 53-55) for a stepwise frequency up-conversion ofthe communication signal, b) amplifying the output signal resulting fromthe frequency up-conversion for obtaining a transmission signal, c)extracting the transmission signal, d) frequency down-conversion of saidextracted transmission signal to the same frequency as the communicationsignal before said up-conversion for creating a feedback signal by usingthe same local oscillators as for the frequency up-conversion forfrequency down-conversion of the transmission signal, and e) adaptingsignal parameters of the communication signal on basis of said feedbacksignal for obtaining a desired character of said transmission signal,characterized in that in step d) frequency signals of at least two ofsaid local oscillators are mixed for combining them into a combinedlocal oscillator frequency signal, which is then used in said frequencydown-conversion stage for frequency down-conversion of the transmissionsignal, thus reducing the number of frequency conversion stages in thetransmission signal path of the frequency down-conversion with respectto the number of frequency conversion stages in the communication signalpath of the frequency up-conversion.
 23. A method according to claim 22,characterized in that the frequency up-conversion in step a) is carriedout by mixing the communication signal in two frequency conversionstages with frequency signals from two said local oscillators (25, 26)and in step d) the transmission signal is only subjected to a frequencydown-conversion once.
 24. A method according to claim 22, characterizedin that in step a) said communication signal is subjected to threefrequency up-conversions, that in step d) firstly frequency signals fromtwo (53, 54) of said local oscillators are mixed for combining them intoa first combined local oscillator frequency signal and then this signalis mixed with the frequency signal from a third (55) of said localoscillators for combining them into a second combined local oscillatorfrequency signal, which is subsequently mixed with the transmissionsignal for frequency down-conversion thereof thus reducing the number offrequency conversion stages in the transmission signal path for saidfrequency down-conversion to one.
 25. A method according to any ofclaims 22-24, characterized in that in step a) the communication signalis firstly mixed with a local oscillator frequency signal in aquadrature modulator (45) and in step d) the transmission signal isfrequency down-converted by being mixed with a combined local oscillatorfrequency signal in a quadrature demodulator (46).
 26. A methodaccording to any of claims 22-24, characterized in that in step a) thecommunication signal is firstly mixed with a local oscillator frequencysignal in an SSB (Single SideBand) mixer and in step d) the transmissionsignal is frequency down-converted by being mixed with a combined localoscillator frequency signal in an SSB mixer.
 27. A method according toany of claims 22-26, characterized in that a frequency signal emitted byat least one of said local oscillators is delayed on the path to saidfrequency down-conversion stage so as to substantially match the delayof this frequency signal from said local oscillator to this stagethrough the local oscillator signal combining step with the delay ofthis signal to this stage through the frequency up-conversion step a)and the power amplifying step b).
 28. A method according to claim 27,characterized in that the frequency signals sent from each localoscillator (25, 26) for local oscillator frequency signal combining instep d) are delayed.
 29. A method according to claim 27 or 28,characterized in that the combined local oscillator frequency signalproduced in step d) is delayed for obtaining said matching.
 30. A methodaccording to any of claims 27-29, charact rized in that frequency signalsent from at least one of said local oscillators is delayed before beingcombined with a frequency signal from another local oscillator forproducing a combined local oscillator frequency signal in step d) forobtaining said delay matching.
 31. A method according to any of claims22-30, characterized in that said communication signal is predistortedfor cancelling out distortions thereof generated in the path of thecommunication signal, such as in a power amplifier (27), for obtaining asubstantially distortion-free transmission signal and this is done instep e).
 32. A method according to claim 31, characterized in that saidtransmission signal is after said frequency down-conversion thereofanalog/digital-converted and said predistortion of the communicationsignal is carried out in the digital domain.
 33. A method according toany of claims 22-32, characterized in that it is carried out on atransmitter comprising at least two communication signal branches (70₁-70 _(N)) for each of which steps a), b) and c) are carried out, andthat steps d) and e) are carried out for each branch for adjusting thegain and/or phase of the transmission signal from the respective branchon the basis of a comparison of a communication signal corresponding toa wanted transmission signal and said feedback signal of each branch ora comparison of feedback signals from all branches.
 34. A methodaccording to claim 33, characterized in that a separate frequencydown-conversion is carried out for each of said branches (70 ₁-70 _(N)).35. A method according to claim 33, characterized in that saidtransmitter has only one frequency down-conversion stage in common toall said branches, and that said frequency down-conversion stage isalternatingly connected to different branches (70 ₁-70 _(N)) for signalparameter adaptation of the communication signals thereof.
 36. A methodaccording to claim 34 or 35, characterized in that the same localoscillators (71, 72) are used for all the branches (70 ₁-70 _(N)) forsaid frequency up- and down-conversion.
 37. A method according to claim36, characterized in that each branch has a separate frequencyup-conversion stage and all frequency up-conversion stages have separatelocal oscillators (71, 72), and that the local oscillators of differentfrequency up-conversion stages are alternatingly connected to the localoscillator combining in step d) of the frequency down-conversion stage.38. A method according to any of claims 33-37, characterized in that thetransmitter comprises an adaptive antenna and each branch (70 ₁-70 _(N))has an antenna (67) for transmitting the respective amplifiedtransmission signal.
 39. A computer program directly loadable into theinternal memory of a computer, comprising software for controlling thesteps of any of claims 22-38 when said program is run on the computer.40. A computer program according to claim 39, provided at leastpartially through a network as the Internet.
 41. A computer readablemedium, having a program recorded thereon, where the program is to makea computer control the steps of any of the claims 22-38.